1. Field of the Invention
The present invention relates to a light information record/playback apparatus, including an optical head having a laser diode provided over a semiconductor substrate with an insulating layer interposed, which can effectively read and write data.
2. Description of the Related Art
A record/playback apparatus is known which can write or read optical information using an optical head. Stated in more detail, an optical type information record/playback apparatus is used which records/plays back information by directing a light spot at an information recording medium. With this type of information record/playback apparatus, a laser diode is employed as a light source of an optical head. A light beam emergent from the laser diode is modulated in accordance with record data. Recording is made by directing the modulated light beam as a light spot on the information recording medium and data playback is made by detecting a reflected light beam from the recording medium by a photodiode.
As the optical head for such an optical type information record/playback apparatus, the adoption of the following structure has been studied. That is, an integrated type optical pickup has been under consideration, including optical semiconductor elements, such as a laser diode and photodiode, provided at a semiconductor substrate. With the integrated type optical pickup (optical head), an optical semiconductor element is formed over the semiconductor substrate and a hard-to-fabricate part is assembled as a mount component part, thus achieving a low-cost compact apparatus.
In the integrated type optical pickup, however, various problems arise from an interference between a semiconductor laser diode (LD) and a photodiode (PD), hard-to-fabricate parts in the manufacture of the optical pickup, etc. For example, Jpn. Pat. Appln. KOKAI Publication No. 59-96789 discloses such problems as set out above. To be brief, when a laser diode and photodiode are formed at one semiconductor substrate, an insulating layer is provided between the semiconductor substrate and the laser diode. This specific structure can prevent an undesirable electrical connection from being made between the laser diode and the photodiode, that is, a problem produced at the time controlling the output of the laser diode. Further, it is possible to ensure the easiness with which the associated components parts are assembled.
In the case where the integrated type pickup is used for an optical magnetic disc, phase variation disc, etc., in particular, of the record/playback apparatus including a compact disc, it is sometimes required that, upon the writing of data, an output of the laser diode be high-speed modulated in accordance with the data. In order to reduce a noise component produced from the laser diode upon reading the data, the laser diode (LD) is sometimes driven with a high-frequency wave added. For the reduction of the noise component, the high-frequency wave to be superimposed is sometimes upon at a very high level of as much as several hundreds of MHz. The high-frequency wave superimposition technique is known in, for example, Jpn. Pat. Appln. KOKAI Publication No. 56-37834.
In the technique as disclosed in Publication 56-37834, if the superimposition of a high-frequency wave is effected, at all times, for noise reduction, the output of the laser diode exceeds its rated level at a high-output time, such as a record/erase time, thus posing an LD's lifetime problem. From this viewpoint, the laser diode is driven in a high-frequency mode at a playback time and stopped at a record/erase time. A switching circuit is provided between the semiconductor laser and a high-frequency wave superimposing circuit and controlled in accordance with a playback mode and record/erase mode. To be specific, a diode is connected across the laser diode and the high-frequency wave superimposing circuit and, by turning this diode ON or OFF, the laser diode is high-frequency driven in an ON/OFF fashion.
If, in this technique, an insulating layer is interposed between the semiconductor substrate and the laser diode, then a problem stays unsolved as will be set out below. That is, if the semiconductor substrate is so interposed, a capacitance is created between the semiconductor substrate and the laser diode due to the presence of the insulating layer and, upon the application of a modulation to the laser diode, provides a problem. Upon the direct application of the modulation signal to the laser diode in this case, a high-frequency current to be applied to the laser diode flows via the insulating layer into the semiconductor substrate due to an action of that capacitance between the laser diode and the semiconductor substrate. It is, therefore, not possible to achieve high-speed modulation because the high-frequency current does not flow into the laser diode side.
Explanation will now be given below briefly about the semiconductor laser diode structure. The semiconductor laser diode has a pn junction as well known in the art. That is, the laser diode is so formed as to have a plurality of p-type semiconductor layers in the surface portion of an n-type semiconductor substrate with an active layer of the laser diode provided as an intermediate layer. By flowing a forward current through the laser diode, a laser beam emerges from the active layer. The laser diode has a pair of opposed electrodes and, of these two electrodes, one is situated on the semiconductor substrate side and the other electrode on a side opposite to said one electrode relative to the active layer.
The laser diode is of such a structure that it is held over the semiconductor substrate of the integrated type optical pickup with the insulating layer interposed in which case any one electrode side is normally used. When a modulation signal is supplied to the laser diode, high-frequency current will be flowed via the insulating layer into the semiconductor substrate side, in spite of the need to be flowed into the laser diode, owing to an action of the capacitance between the laser diode and the semiconductor substrate of the integrated type optical pickup. As a result, almost no high-frequency current flows through the layer diode, thus failing to achieve high-speed modulation.
It is, therefore, not possible to write data at high speeds and hence to reliably read it because there is no adequate superimposition of the high frequency wave.